An atmospheric resid (“AR”) is a heavy oil that contains about 30 wt. % or more of species having an atmospheric boiling point of about 540° C. and higher. Vacuum resid (“VR”) is a heavy oil that contains about 60 wt. % or more of such species. Such heavy oils can be hydroprocessed in order to remove impurities such as sulfur, for conversion into a valuable light oil. Generally, hydroprocessing is used to remove micro carbon residue (“MCR”), sulfur, various metals, nitrogen, and asphaltene.
Asphaltene is an aggregate of condensed aromatic species, which are dissolved or suspended in the heavy oil. In the event that asphaltene is cracked excessively during hydroprocessing, cohesion of the cracked fragments occurs to form granular species, e.g., sludge and/or sediment.
Sediment is a deposit which can be measured by testing a sample according to the Shell Hot Filtration Solid Test (SHFST) (see van Kerkvoort et al., J. Ins. Pet., 37 pages 596-604 (1951)). Sediment generally comprises species having an atmospheric boiling point of about 340° C. or more, and when collected from flash drum bottoms in a refining process, comprise about 0.19 to about 1 wt. %, based on the weight of the bottoms. Sediment can settle, during oil refining, in equipment such as heat exchangers, reactors and downstream filters, and undesirably restrict flow.
Especially in the hydroprocessing of a heavy oil containing a large amount of vacuum resid, sediment would be formed in even greater abundance and, therefore, it would be desirable to improve the hydroprocessing catalyst and hydroprocessing methods to achieve the desired level of impurity removal while keeping the formation of sediment as low as possible.
It is known (i.e., conventional) in the hydroprocessing of heavy hydrocarbons that if catalysts having different functions are used in combination, they can exhibit improved performance over that exhibited by individual catalysts. When a combination of independently selected catalysts having specific pore sizes is used in a two-stage hydroconversion process, a vacuum resid containing heavier fractions can be highly hydroprocessed to produce a light oil with an economically high added-value, while the generation of sediment is suppressed.
Accordingly, conventional heavy oil processing generally comprises two catalytic steps. In the first step, a catalyst with activity for cracking asphaltene and, optionally, for removing metals is used to decrease the asphaltene content to inhibit the formation of sediment. A second step (final step) uses a catalyst having a high desulfurization activity to hydrodesulfurize the product of the first step to produce a desulfurized oil.
For example, JP 7-65055B, (“'055”, Gazette), discloses a hydroprocessing method for converting the heavy portion of hydrocarbon oil containing sulfur impurities and metallic impurities at least in two steps. This technology relates to a hydroprocessing method using a catalyst containing about 0.1 to about 5 wt. % of a metal oxide for hydrodemetallization in the first step and using a hydrodesulfurization catalyst containing about 7 to about 30 wt. % of a metal oxide in the subsequent second step. In case of this method, it is said to be beneficial that demetallization and hydrocracking are carried out in the first step and that the resid is processed by desulfurization in the second step. The reference discloses a combination of catalysts where the first step catalyst has a “sea urchin” structure and the second step catalyst has an alumina-cohering state. However, because the amount of the catalyst used in the first step is small, the desulfurization and hydrogenation functions decline though the demetallization function required in the first step is improved. Therefore, in the second step, since a high desulfurization function is needed, a greater amount of catalyst is used compared to the first step. Since a high desulfurization function is necessary in the second step, a higher temperature is used which leads to a higher cracking rate and, consequently, asphaltene is precipitated.
Patent application JP 8-325580A, (“'580”, Gazette), discloses a catalytic hydroconversion method for a heavy feedstock. The '550 reference discloses a method using a first step catalyst, in which a carrier material selected from alumina, silica and a combination thereof is loaded with a total of about 2 to about 25 wt. % of oxides of active metals selected from cadmium, chromium, cobalt, iron, molybdenum, nickel, tin, tungsten, and combinations thereof. Reaction conditions in the first step include a reaction temperature of about 438 to about 468° C., a hydrogen partial pressure of about 105 to about 245 kg/cm2, and a space velocity of about 0.3 to about 1.0 (Vf/hr/Vr). A similar catalyst is used in the second step, where reaction conditions include a reaction temperature of about 371° C. to about 427° C., a hydrogen partial pressure of about 105 to about 245 kg/cm2, and a space velocity of 0.1 to 0.8 (Vf/hr/Vr).
The '580 patent application discloses an improved catalytic hydroconversion method (i.e., H-Oil™, available from AXENS) for a heavy hydrocarbon upgrading. The improvement addresses the efficient use of catalysts, the product quality, and the management of unreactive residue by means of re-circulation. The '580 patent discloses a higher reaction temperature and lower catalyst activity in the first step, and a lower reaction temperature and higher catalyst activity in the second step. Disadvantageously, the high temperature reaction of the first step leads to the thermal condensation of asphaltene and molecular fragments such as petroleum. Such asphaltene and resins can lead to undesirable coking of the second-stage catalyst. Moreover, according to the '580 disclosure, the second-stage catalyst is not well suited for preventing the cohesion and precipitation of asphaltene formed in the second step.
Patent application JP 6-53875B, (“'875”, Gazette), is also directed towards a multi-step, catalytic heavy hydrocarbon conversion method. In the first step, a fixed bed or an ebullated bed reactor is operated at a reaction temperature of about 415 to about 455° C., a hydrogen partial-pressure of about 70 to about 211 kg/cm2, and a space velocity of about 0.2 to about 2.0 (Vf/hr/Vr). In the second step, an ebullated bed reactor is used at a reaction temperature of about 415 to about 455° C., a hydrogen partial pressure of about 70 to about 211 kg/cm2, and a space velocity of about 0.2 to about 2.0 (Vf/hr/Vr). The catalyst support contains alumina, silica, and mixtures thereof. An oxide of a catalytically active metal selected from cadmium, chromium, cobalt, iron, molybdenum, nickel, tin, tungsten, and mixtures thereof, is present on the support.
The '875 patent discloses the recirculation of vacuum bottoms to achieve a high cracking rate, without regard for asphaltene cohesion. detrimental to high cracking rate operation is disclosed. The aim of the first stage is catalytic demetallization, not the prevention of asphaltene precipitation.
Therefore, there is a need in the art for a heavy hydrocarbon hydroconversion process that inhibits the formation of sediment detrimental to operation in the hydrocracking of heavy oil while achieving sufficient levels of desulfurization and cracking.
There is also a need for an effective hydroprocessing method using a combination of catalysts for the hydroprocessing of a heavy hydrocarbon oil containing a large amount of impurities such as sulfur, micro carbon residue (“MCR”), metals, nitrogen, and asphaltene.
There is also a need for a hydroconversion process with improved asphaltene cracking, and a combination of catalysts, which enables production of highly desulfurized oil while decreasing the sediment formation with an increase of conversion rate.